Plates for osteosynthesis or arthrodesis are often fixed to bone substantially parallel to a longitudinal bone axis using screws or other fixation elements. Such plates are firmly fixed to a plurality of bone parts or fragments to prevent their movement relative to each other.
Spherical-head screws cooperating with radiused apertures in plates have been used to bring the plate into compression onto a bone until the friction of the plate on the bone stabilizes the assembly. These assemblies allow screws to be implanted during a surgical operation within the plate aperture at multiple angles relative to the plate to compress separated bone parts or fragments. The shortcoming of these systems is their low resistance to compression stresses exerted parallel to a plane generally defined by the plate.
A second generation of assembled screw-and-plate systems called monoaxial-locking and polyaxial-locking systems have relied on a threaded engagement of a screwhead with a plate aperture such that the strength of the assembly no longer depends on the compression of the plate against the bone. These systems allow for the assembly to take place away from the bone section being repaired and, in some instances, the possibility of selecting an angle of screw implantation during an operation while achieving strength sufficient for postoperative stresses.
Inserts have been added to monoaxial or polyaxial screw-and-plate systems in which the insert has a threaded engagement with any of the plate and the screw. In some such systems, a screw has been threaded into an insert to lock the insert, by way of friction, to a plate at an angle corresponding to a desired angle of implantation for the screw. To enhance the locking of these inserts, an outer surface of inserts has been given radial offsets. Such offsets have extended along an entire arcuate surface within a plane through a longitudinal axis defined by the inserts which creates friction that diminishes the control of the insert when attempting to set the desired insertion angle.
Thus, there exists a need for a better way in which to align fasteners for fixation within plates.